The invention relates to a molded part made of electrically conductive ceramic, in particular filter parts, and the molded part is a flow-through part because of pores and has contact zones for attaching electrodes. The invention further relates to a process for producing contact zones for attaching electrodes to molded parts that are flow-through parts because of pores and are made of electrically conductive ceramic.
Ceramic molded parts are being increasingly proposed as filter elements, but also as supports for catalysts. They are especially suitable for filtering hot gases, since they are extraordinarily temperature-resistant. They are thus to be used, among other things, for removing rust particles in the exhaust gas of diesel engines. Here, silicon carbide has turned out to be an especially suitable ceramic compound, since it is chemically stable and has a great heat conducting capacity and is resistant to temperature and thermal shock (see EP-A-O 796 830, DE-C-41 30 630, EP-A-O 336 883, WO 93/13303).
For use in diesel engines, it is important that the rust particles deposited on or in the molded part be removed at regular intervals, so that the flow resistance in the walls of the molded part do not become too high. A known cleaning method consists in heating the molded part so hot by applying an electrical current that the rust particles ignite and burn up. For this purpose, the molded part must be connected to electrodes by which the electrical current can be introduced into the molded part.
The connection of the electrodes to the molded part must be temperature resistant and it must be able to withstand the strong thermal shocks and the mechanical stresses. Further, as little contact resistance as possible must be sought. Achieving these goals creates considerable difficulties because of the porous and rough surface of such molded parts. Known solutions, as they come out of U.S. Pat. No. 4,505,107, U.S. Pat. No. 4,535,589, U.S. Pat. No. 4,897,096 and U.S. Pat. No. 5,423,904, for example, are not satisfactory.
The object of the invention is thus to configure the contact zones on molded parts of the above-mentioned type so that a secure attachment of the electrodes with low contact resistance is guaranteed.
This object is achieved according to the invention by infiltrating an electrically conductive metal into the pores of the contact zones. To the extent that the ceramic consists of a metallic ceramic compound, a metal that is the same as the metal of the ceramic compound should preferably be used for the infiltration. Understood here as a metallic ceramic compound in the sense of this description is a compound that has at least one metal atom.
It has turned out that by infiltrating an elementary metal, contact zones can be produced that are distinguished by temperature resistance, a stable surface favorable for attaching electrodes, and low contact resistance. Over the entire temperature range, the thermal expansion of the contact zones does not differ significantly from the rest of the areas of the molded part, i.e., there are no thermal stresses that could damage the contact. Further, the contact zones are corrosion resistant.
Under the concept metalxe2x80x94in elementary form and as a component of a ceramic compoundxe2x80x94not only metallic, but also metal-like elements are meant in the sense of this description, for example metalloids. Involved here, besides silicon, are wolfram, tantalum, hafnium, zirconium, titanium, molybdenum, etc. The molded part itself can consist, e.g., of metal carbides (SiC, WC, TaC, HfC, ZrC, TiC), metal nitrides (ZrN, TiN), metal borides (TiB2, ZrB2) or silicides (MoSi2) Silicon carbide has proven especially useful.
To prevent the metal to be infiltrated from spreading over the entire molded part, it is proposed that the contact zones have additional contact layers made of an electrically conductive ceramic into which the metal is infiltrated, and suitably the ceramic of the contact layers is the same as that of the molded part. Too great a penetration of the metal into the molded part itself can be prevented above all by making contact layers with finer pores than the molded part itself. In this way, the capillary action in the contact layers is greater than in the molded part. This capillary action limits the infiltration of the metal into the molded part, and it can be quite advantageous if the metal is partly infiltrated beyond the contact layers into the molded part. The contact layers can be at least partly pressed into the molded part. Such contact layers yield smooth and highly conductive contact surfaces.
According to the invention it is further provided that the molded part has a core part and connecting parts that are connected to one another, and the contact zones are on the connecting parts. The infiltration of the metal can thus be limited to the connecting parts by subjecting them alone, separately from the core part, to infiltration conditions and then subsequently connecting them to the core part. Preferably the core part and the connecting parts have the same ceramic base. The connecting parts can be used as end parts. The division of the molded part into core part and end part has the additional advantage that the enclosures for the flow channels going through the end parts can be provided simultaneously with the production of the end part, e.g., by suitable molding of the end parts.
But it is not necessary that the connecting parts simultaneously form the end parts. They can be placed between the parts of a divided core part to achieve other flow paths. In both cases, it is suitable if the connecting parts are made as a continuation of the core part, i.e., especially flow-through channels in the same arrangement with the same cross sections. But alternatively, the connecting parts could also be attached laterally to the core part. Of course numerous core parts can be connected in parallel or in series to each other by the connecting parts.
The connection of the connecting parts to the core part can be performed, for example, by a material joining using active soldering (see M. Boretius, E. Lugschneider, Active Solderingxe2x80x94Material Joining of Ceramic Substances to Each Other and to Metal, VDI Berichte [Reports] No. 670, 1988). But instead there is also the possibility of materially connecting the connecting parts to the core part by a connecting layer made of electrically conductive ceramic, and suitably here also the ceramic of the molded part and of the connecting layer is the same. This yields secure, electrically conductive connection between the parts. Likewise, it can be recommended to make the connecting layer with pores finer than those of the molded part, to be able to infiltrate electrically conductive metal also into the connecting layer. In doing so, the contact resistance between the parts is kept low. Here it is entirely desirable that the metal infiltrated into the connecting parts is also partially infiltrated into the connecting layer.
According to another feature of the invention, it is provided that at least one connecting part has an attachment with contact zones. In particular, if the connecting parts form continuations of the core part, in this way the contact zones for attaching the electrodes can be relocated from the xe2x80x9chotxe2x80x9d area of the core part to the cooler areas. The molding of the attachments can be done in any way according to the above-mentioned purpose. It is proposed that the attachments, like the connecting parts, be made of an electrically conductive ceramic with electrically conductive metal infiltrated into them, and specifically suitably of the same ceramic from which the connecting parts are also made, and infiltrated with the same metal. Connecting parts and attachments here can also be made of a uniform fitting. If this is not possible for reasons of production technology, the attachments can also be produced separatelyxe2x80x94suitably in the same work step as the connecting partsxe2x80x94and then be connected to the respective connecting part. The same processes that were described above for the connection of connecting parts and core part are involved here.
The infiltration of the electrically conductive metal into the pores of the contact zones is performed by heating the relevant metal to at least its melting point, preferably even to a higher temperature, to perform a reaction burn. The temperature can also be so high that the infiltration is performed partly or completely in the vapor phase. If silicon is used as the metal, the temperature should be at least 1400xc2x0 C., preferably 1600xc2x0 C. The infiltration is performed in a vacuum or protective gas, to exclude the effect of oxygen.
The metal to be infiltrated can be applied in elementary form to the contact surfaces before the infiltration procedure, for example as a powder, paste, platelets, foil, or the like. But also possible is the use of a mixture of substances in which the metal is present in hyperstoichiometric form and is partially converted into an electrically conductive ceramic by heating to reaction temperature.
Alternatively to this it is provided that the contact zones are provided with a contact layer that contains substances for forming an electrically conductive ceramic compound, that these substances are converted into the ceramic compound by heating to at least reaction temperature and with the formation of fine-pored contact layers, and that the metal is infiltrated simultaneously or afterwards into the pores of the contact layers. The contact layer can be present for example in paste form and there can also be an excess of the metal to be infiltrated. Instead of this, the metal can also be applied in powder, paste and platelet form to the outside of the contact layer and then be subjected to infiltration conditions. Here the conversion is to be performed so that a porosity is achieved that is less than that of the molded part. This largely prevents the infiltration of the metal into the molded part itself. Further, it can be desirable that the metal be used in an amount such that it still infiltrates partially into the molded part.
The process according to the invention can be configured so that first a core part and a connecting part are produced, the metal is infiltrated only into the connecting parts, separately from the core part, and core and connecting parts are connected to one another. In this way it is assured that the infiltration of the metal remains limited to the it connecting parts. The connection of the connecting parts to the core part can be done by active soldering, high temperature soldering or diffusion welding. But suitably there is also an alternative process in which a connecting coating is applied to the surfaces of connecting parts and/or core part to be connected and the coating contains substances for the formation of an electrically conductive ceramic compound, and these substances are converted into the ceramic compound by heating to at least reaction temperature to form an electrically conductive connecting layer. Suitably, the conversion of the substances into the ceramic compound is simultaneous with the infiltration of the metal into the pores of the contact zones. Here it is favorable for the compound if substances are used for the connecting coatings that convert into the same ceramic compound as that of the molded part.
With this type of connection there is also the possibility of infiltrating an electrically conductive metal into the connecting layer, and it is also suitable if, in the conversion of the connecting layer, a lower porosity is achieved than that of the molded part. To the extent that a metallic ceramic compound is used, it is advantageous if a metal is used for the infiltration that is the same as the metal of the ceramic compound that forms the connecting layer. The infiltration of the metal can be performed by having the metal present in the connecting coating in hyperstoichiometric form. In doing so, an amount of metal is to be used such that it also partially infiltrates the molded part.
Finally it is provided according to the invention that the electrodes are connected to the contact surfaces by active soldering, high temperature soldering or diffusion welding. The thermal expansion of the electrodes should be matched to that of the molded part by geometric or material-specific factors.